Final answer:
Virulence is measured by ID50, which is the dose needed to cause symptoms in 50% of hosts. The correct answer to the question is B) id50; cause symptoms in 50%.
Step-by-step explanation:
Virulence is measured by ID50, or the dose needed to cause symptoms (not necessarily death) in 50% of the host population. The correct answer to the question is B) id50; cause symptoms in 50%. The ID50 is used to determine the median infectious dose, which is the concentration of a pathogen that will produce an active infection in 50% of the test animals inoculated. Conversely, the LD50, which stands for lethal dose, 50%, is the amount of a pathogen that kills 50% of infected test animals. For example, if pathogen A has an ID50 of 50 particles, it is more virulent than pathogens B and C with ID50s of 1,000 and 1 × 106 particles, respectively.